1. Field of the Invention
The present invention relates generally to a liquefied natural gas (LNG) system for a natural gas vehicle engine. More particularly, the present invention relates to a liquefied natural gas system for increasing the pressure in a cryogenic fluid storage tank.
2. Related Art
Cryogenic liquids, such as liquefied natural gas (LNG), nitrogen, oxygen, carbon dioxide, hydrogen, and the like, are substances that normally exist as gases under normal atmospheric conditions, but are liquids at very low temperatures. Special vessels and systems are used to store and transfer cryogenic liquids because of the difficulty in maintaining the extremely cold temperatures. Such vessels typically include a double walled vessel having insulation and vacuum in the annular space. While the vacuum provides an effective insulation, the insulation is not perfect and heat penetrates the vessel walls. When heat is added to the cryogenic liquid, a portion of the liquid returns to the gaseous state, and the temperature and pressure of the contents of the tank are raised. The increased amount of gas within the vessel increases the internal pressure.
Conversely, when either liquid or vapor is withdrawn from the tank, the decreased volume of liquid and vapor in the tank causes the pressure to reduce. As pressure is reduced below the boiling point of the liquid, a portion of the liquid boils off. The boiling of the liquid reduces the temperature of the liquid, which in turns reduces the boiling point of the liquid. The result is that the liquid and vapor in the tank reach a new equilibrium state at a lower temperature and pressure. As liquid and vapor continues be withdrawn, the boiling of small amounts of liquid causes the saturation pressure and temperature to be lowered. The saturation pressure is the pressure at which the liquid is at its boiling point at a given temperature.
In common practice today, there are two methods for transferring the LNG fuel from the vehicle tank to the engine of the natural gas powered vehicle. The first is to use a pump to physically pressurize the LNG and move it to the engine. The second method is to use the pressure of the LNG in the vehicle fuel tank to move the fuel to the engine.
The problem with the systems that utilize tank pressure to move the fuel to the engine is that most engines require a minimum pressure of the fuel in order to be able to generate full rated power. If the fuel is at too low a pressure, or if the pressure in the tank is reduced due to fuel being removed, then it is possible that the engine power will be reduced or the vehicle will become inoperable.
A method for building pressure in typical cryogenic tanks is to circulate the cryogenic liquid into a heat exchanger, where the liquid is vaporized, raising the pressure in the tank. This typical pressure building system uses the liquid head caused by the depth of the fluid to generate flow through the circuit. The problem in cryogenic tanks, especially small horizontal cryogenic tanks used as vehicle tanks, is that there is only a very small liquid depth available in the tank to generate liquid head to drive a pressure building circuit. Pressure drop in the heat exchanger and piping components is large enough that the liquid head in the tank cannot overcome the resistance to flow, resulting in no flow through the pressure building circuit, and therefore no resulting pressure increase in the tank. Because of this there is a need for a different method for driving liquid through a pressure building system.